Plastic optical fiber, optical fiber cable, optical fiber cable with plug, method for producing methyl methacrylate based polymer and method for producing plastic optical fiber

ABSTRACT

This invention provides a process for manufacturing a methyl methacrylate polymer comprising the steps of feeding a monomer containing at least 80 wt % of methyl methacrylate and a radical polymerization initiator represented formula (III) to a reactor; polymerizing the material at a polymerization temperature of 110 to 160° C. under the conditions satisfying particular equations between an initiator concentration and a polymerization temperature; feeding a reaction mixture taken out from the reactor to a devolatilization step (feeding step); and separating and removing volatiles from the reaction mixture (devolatilization step). A methyl methacrylate polymer having adequately good optical properties and a plastic optical fiber having improved transmission performance can be prepared according to this invention.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a plastic optical fiber, an opticalfiber cable and an optical fiber cable with a plug which have a reducedlight-transmission loss; processes for manufacturing a methylmethacrylate polymer with improved optical properties; and processes formanufacturing a plastic optical fiber.

[0003] 2. Description of the Prior Art

[0004] A methyl methacrylate polymer may be prepared by a polymerizationprocess such as suspension polymerization, solution polymerization andbulk polymerization; bulk polymerization and solution polymerization areadvantageous for reducing light-scattering matters such as dusts andimpurities in a polymer. In particular, bulk polymerization is mostadvantageous because solution polymerization requires removing not onlyunreacted monomers but also a solvent. JP-B 5-32722 has disclosed aprocess for manufacturing a plastic optical fiber comprising the stepsof preparing a methyl methacrylate polymer with improved opticalproperties and containing a reduced amount of light-scattering matterssuch as dusts and impurities and light-absorbing matters such asperoxides and oligomers, and then forming a plastic optical fiber usingthe polymer as a core component. In the process, the polymer is preparedusing a radical polymerization initiator represented by formula (I)(hereinafter, referred to as an “initiator I”) such that there is arelationship between an initiator concentration and a polymerizationtemperature satisfying a particular condition.

[0005] However, since the initiator I is used in the process of JP-B5-32722, a polymer obtained has a terminal C₅H₁₁moiety different from amethyl methacrylate unit, which causes an uneven molecular structure anddeteriorated optical properties in the polymer. When using this polymerfor preparing an optical fiber, the optical fiber exhibits inadequatetransmission performance. Thus, there has not been provided a processfor manufacturing a methyl methacrylate polymer with adequately improvedoptical properties, or for manufacturing a plastic optical fiberexhibiting satisfactory transmission performance.

SUMMARY OF THE INVENTION

[0006] An objective of this invention is to provide processes formanufacturing a methyl methacrylate polymer having adequately improvedoptical properties and for manufacturing a plastic optical fiberexhibiting satisfactory transmission performance.

[0007] This invention provides a process for manufacturing amethacrylate (co)polymer comprising conducting polymerization whilefeeding a monomer (mixture) containing at least 90 wt % in total of atleast one methacrylate monomer and a radical polymerization initiatorrepresented by formula (II) into a reactor, where an initiatorconcentration and a polymerization temperature satisfy a relationshiprepresented by equations (1) to (4) and the polymerization temperatureis not less than 110° C. and not more than 160° C.;

ln (A)≦105.4−45126/B   (1)

ln (A)≦2545.2/B−15.82   (2)

ln (A)≧225.9−102168.8/B   (3)

ln (A)≧1300.0/B−15.74   (4)

[0008] wherein A is an initiator concentration (a molar ratio of theinitiator/the monomer); B is a polymerization temperature (° K); and inis a symbol for a natural logarithm;

[0009] wherein R is alkyl or fluoroalkyl.

[0010] This invention also provides a process for manufacturing anoptical fiber comprising preparing a (co)polymer by the above processfor manufacturing a methacrylate (co)polymer comprising conductingpolymerization while feeding a monomer (mixture) containing at least 90wt % in total of at least one methacrylate monomer and a radicalpolymerization initiator represented by formula (II) into a reactor,which further comprises a feeding step of feeding a reaction mixturetaken out from the reactor to a devolatilization step and adevolatilization step of separating and removing volatiles from thereaction mixture; and feeding the thus obtained (co)polymer and anotherpolymer having a different refractive index to a multi-componentspinning nozzle for spinning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows the relationships between a concentration of aninitiator III and a polymerization temperature in this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] In this invention, polymerization is conducted using the radicalpolymerization initiator represented by formula (II) (hereinafter,referred to as an “initiator II”), and for preparing a methylmethacrylate (co)polymer, it is preferable to conduct the polymerizationusing the radical polymerization initiator represented by formula (III)(hereinafter, referred to as an “initiator III”). Herein, a “radicalpolymerization initiator” is simply referred to as an “initiator”.

[0013] wherein R is alkyl or fluoroalkyl.

[0014] The initiator III may be decomposed as illustrated in thefollowing reaction equation to give a radical IV.

[0015] Since the radical IV has the same structure as a methylmethacrylate structural unit, if for example in the polymerization ofmethyl methacrylate (hereinafter, referred to as “MMA”) the initiatorIII is used, a polymethyl methacrylate (hereinafter, referred to as“PMMA”) consisting of the entirely same structural units to itsmolecular ends is obtained. When polymerizing MMA with another monomer,the molecular ends of a copolymer produced are composed of the samestructural unit to MMA which is one copolymerizing component of themonomer mixture. In other words, a polymer prepared using the initiatorIII has a uniform molecular structure. Furthermore, when polymerizing amethacrylate monomer other than MMA, the structural unit of themolecular end of a product obtained has a similar structure to thestructural unit of the monomer. For this reason, a methacrylate polymerprepared using the initiator III can exhibit improved optical propertiesand, when used in an optical fiber, can provide the optical fiber havingimproved transmission performance.

[0016] An initiator generating the radical IV is not known among organicperoxides, but only the initiator III is known among azo initiators. Theinitiator III is, therefore, used in this invention. Decomposition ratesfor the initiator III at different polymerization temperatures arelisted in, for example, the technical bulletin of Wako Pure Chemicals.

[0017] When preparing a polymer mainly comprising a structural unit ofanother methacrylate monomer, it is also preferable to select aninitiator such that a radical generated from the initiator has the samestructure as the structural unit of the monomer. For example, whenpreparing a 2,2,3,3-tetrafluoropropyl methacrylate polymer, it ispreferable to use the initiator II where R in formula (II) is2,2,3,3-tetrafluoropropyl.

[0018] In this invention, a chain transfer agent, preferably an alkylmercaptan having 3 to 6 carbon atoms may be used for adjusting themolecular weight of a polymer produced. Residual mercaptan compounds maycause discoloration in thermal hysteresis during molding for amethacrylate polymer. It is, therefore, desirable to fully remove themduring a devolatilization step. A mercaptan having 3 to 6 carbon atomsis easily handled because it is liquid at an ambient temperature and hasa relatively higher vapor pressure, permitting us to remove most of themduring the devolatilization step. As a result, an industriallyadvantageous and satisfactorily transparent methacrylate polymer with anextremely less amount of impurities can be prepared.

[0019] In this invention, it is preferable to conduct polymerization bya bulk or solution polymerization process. Further, it is preferable toconduct these polymerization continuously. Particularly, continuous bulkpolymerization is most preferable since it does not require a solventremoving step and can provide a polymer with a minimum amount ofimpurities.

[0020] This invention will be described in detail. In the followingdescription, the initiator III is used as the initiator II, but aninitiator II in which R is other than methyl may be also used in asimilar manner.

[0021] For the initiator III which is preferably used in this invention,it is preferable to remove foreign materials before use. For removingforeign materials, the initiator III is, for example, filtrated with aknown filter. Since the initiator III is liquid at an ambienttemperature, foreign materials can be readily removed. When using acommercially available initiator III, it usually contains materialsother than the initiator III. Herein, an initiator purity, i.e., aninitiator III concentration in the product, is preferably at least 95 wt%, more preferably at least 97 wt %. As used herein, the term “foreignmaterials” means dirts, dusts and so on. In particular, it is preferableto remove foreign materials having a size of 0.02 μm or more byfiltration. As used herein, the term “materials other than the initiatorIII” means residual compounds contained in an initiator III product suchas starting materials for synthesis and byproducts.

[0022] A monomer (mixture) used in this invention is a monomer (mixture)containing at least 90 wt % of a methacrylate monomer, preferably MMA ora mixture of at least 50 wt %, preferably at least 80 wt % of MMA andother monomer(s). Herein, one kind of methacrylate monomer or two ormore kinds of methacrylate monomers may be used. When using two or morekinds of methacrylate monomers, the total amount of them is regarded asa content for a methacrylate monomer. Other monomers which may becombined with MMA are preferably, but not limited to, a variety of(meth)acrylates such as fluoroalkyl methacrylates, benzyl methacrylateand methyl acrylate.

[0023] When using a chain transfer agent in this invention, the abovealkyl mercaptans having 3 to 6 carbon atoms are preferably used. Suchalkyl mercaptans include n-propyl mercaptan, n-butyl mercaptan, t-butylmercaptan and n-hexyl mercaptan. Among them, n-butyl mercaptan ispreferable.

[0024] Such an initiator, a mercaptan and a monomer are fed in a reactorto initiate polymerization. In the process, they are preferably fed intothe reactor after fully removing dissolved oxygen from the reactor by,for example, introducing inactive gas such as nitrogen gas ormaintaining the system under a reduced pressure for a certain period. Inaddition, an initiator is usually fed by blending it just before thereactor with the other components to prevent these blended materialsfrom being polymerized before introduction into the reactor.

[0025] The initiator, the mercaptan and the monomer fed into the reactorare blended by stirring, during which an inert gas such as nitrogen ispreferably introduced into the reactor to pressurize the system to thevapor pressure of the reaction or higher.

[0026] In this invention, a known reactor may be used. It is preferableto use a reactor equipped with a jacket for internal heating or cooling.A known agitator may be used; preferably, a double-helical ribbon bladeor a Max Blend blade (Sumitomo Jukikai Kogyo Inc.). The agitator may bepreferably operated with an agitation power within the range of 1 to 5kW/m³.

[0027] The concentration of the initiator III fed into the reactor ispreferably selected within the range satisfying the following equations(1) to (4) in order to conduct polymerization economically and stably:

ln(A)≦105.4−45126/B   (1)

ln(A)≦2545.2/B−15.82   (2)

ln(A)≧225.9−102168.8/B   (3)

ln(A)≧1300.0/B−15.74   (4)

[0028] wherein A is an initiator concentration (a molar ratio of theinitiator/the monomer); B, is a polymerization temperature (° K); and lnis a symbol for a natural logarithm.

[0029] When conducting solution polymerization by further feeding aninert solvent into the reactor, the concentration of the initiator IIIis preferably selected within the range satisfying the followingequations (5) to (8):

ln{A×(1−C)⁵}≦105.4−45126/B   (5)

ln{A×(1−C)⁵}≦2545.2/B−15.82   (5)

ln{A×(1−C)⁵}≧225.9−102168.8/B   (7)

ln{A×(1−C)⁵}≧1300.0/B−15.74   (8)

[0030] wherein C is the concentration of the inert solvent (the amountof the inert solvent (g)/the total amount of the monomer, the initiator,the chain transfer agent and the inert solvent fed into the reactor (g))and A, B and ln are as defined for equations (1) to (4).

[0031] If the concentration of the initiator III is higher than thelimit defined by equation (1) or (5), a reaction mixture may adhere tothe reactor or may be postpolymerized outside of the reactor. It maycause an uneven molecular weight or may tend to generate foreignmaterials such as gel, resulting in deteriorated optical properties in apolymer obtained.

[0032] If the concentration of the initiator III is higher than thelimit defined by equation (2) or (6), it becomes difficult tohomogeneously blend the initiator and the monomer in the reactor,leading to poor operation stability. As a result, it may also cause anuneven molecular weight and foreign materials such as gel, resulting indeteriorated optical properties in a polymer produced.

[0033] If the concentration of the initiator III is lower than the limitdefined by equation (3) or (7), oligomers mainly comprising dimers maybe increased. Thus, for example, when a polymer obtained is used for aplastic optical fiber (hereinafter, referred to as an “optical fiber”),an absorption loss in the optical fiber may be increased.

[0034] If the concentration of the initiator III is lower than the limitdefined by equation (4) or (8), oligomers mainly comprising dimers maybe increased and a polymer yield may be reduced, leading to reduction inan economic efficiency.

[0035]FIG. 1 graphically shows the condition defined by equations (1) to(4). An initiator concentration and a polymerization temperature, i.e.,a temperature of a reaction mixture during polymerization, are selectedwithin the hatched range delimited by these four lines in the graph.

[0036] A polymerization temperature may be selected within the range of110 to 160° C. If the polymerization temperature is higher than 160° C.,dimers may be increased. The dimers cannot be completely separated by ausual devolatilization process, so that they may remain in a polymer,causing density fluctuation and thus deteriorated optical properties inthe polymer. When attempting to remove the dimers for preventing theabove problems, a reaction mixture must be heated to an elevatedtemperature during removing volatiles, which may cause discoloration inthe polymer. The polymerization temperature is preferably 150° C. orlower, more preferably 140° C. or lower, further preferably 130° C. orlower for ensuring stabler preparation of a polymer having more improvedoptical properties.

[0037] On the other hand, a polymerization temperature lower than 110°C. may cause an uneven molecular weight of a polymer produced, leadingto a reduced polymer yield.

[0038] A polymerization temperature is controlled to be maintained adesired constant temperature by, for example, adjusting a jackettemperature in the reactor or a temperature of a fed monomer.

[0039] When using continuous bulk polymerization as a polymerizationprocess, a polymer content in a reaction mixture within a polymerizationzone is preferably 30 wt % or higher for minimizing formation of dimersin the reaction mixture. For achieving an even molecular weight of apolymer in the reaction mixture and improving controllability of apolymerization temperature of the reaction mixture, a polymer content inthe reaction mixture is preferably 70 wt % or less, more preferably 60wt % or less. For stabler polymerization, a polymer content at apolymerization temperature of 140° C. or lower is preferably 50 wt % orless. A polymerization zone is herein a region where the initiator IIIand the monomer fed into the reactor are substantially homogeneouslystirred and blended so that polymerization proceed to provide a reactionmixture.

[0040] For adjusting a molecular weight of a polymer, a chain transferagent such as, generally, a mercaptan is added into a reactor. However,since it is necessary to remove most of the unreacted mercaptan during adevolatilization step for providing a methacrylate polymer havingimproved transparency, there has been a problem that a mercaptan havinga higher number of carbon atoms has a lower vapor pressure, leading toincreased duty in the devolatilization step. It is, therefore,preferable in this invention to use an alkyl mercaptan having 3 to 6carbon atoms as a mercaptan having a relatively higher vapor pressure.If the carbon number is less than 3, handling the mercaptan becomesdifficult because it can be easily vaporized at an ambient temperature.If the carbon number is more than 6, a vapor pressure is lower so thatduty in the devolatilization step may be increased. There are nolimitations for the amount of a mercaptan having 3 to 6 carbon atoms,and an amount appropriate for adjusting a molecular weight may be used.A particularly preferable chain transfer agent is n-butyl mercaptan.n-Butyl mercaptan has a boiling point almost equal to that of methylmethacrylate. n-Butyl mercaptan is, therefore, not separated from methylmethacrylate monomer and can be recovered as a solution in the monomer,even after removing materials having higher and lower boiling pointsfrom volatiles by distillation for reutilizing in the polymerizationzone the volatiles recovered in the devolatilization step. Thus, usingn-butyl mercaptan as an alkyl mercaptan, it can be reutilized.

[0041] However, volatiles recovered by a devolatilization processcommonly used, occasionally contain unknown coloring materials exceptfor a monomer and a mercaptan. Therefore, for reutilizing the volatiles,it is preferable to purify the volatiles by a process described laterand extract the monomer (mixture) from the volatiles removed in adevolatilization step described later for reutilizing. The monomer(mixture) extracted in the volatile purification step may be recycledinto the reactor or may be used in a different utility as a usualmonomer (mixture).

[0042] For improving productivity of a polymer and minimizingcontamination with dusts and/or polymer gels as much as possible in thepresent invention, it is preferable to continuously conductpolymerization, i.e., to continuously feed an initiator III, a monomerand preferably further a mercaptan compound selected from alkylmercaptans having 3 to 6 carbon atoms into a reactor for polymerizationwhile continuously taking out a reaction mixture from the reactor. Inthe process, an average residence time of the reaction mixture in apolymerization zone is preferably 1 to 6 hours, more preferably 2 to 6hours.

[0043] After polymerization, preferably the reaction mixture taken outfrom the reactor is continuously fed with a known means such as a pumpto a devolatilization step.

[0044] There are no limitations for a devolatilization process, and anyknown process can be employed. For example, volatiles can be removed byfeeding the reaction mixture to a vent-type extruder. Although higherinternal temperature of the extruder may be more effective for removingvolatiles, it may cause deteriorating a polymer by staining the polymerobtained after removing the volatiles. It is, therefore, preferable toselect the lowest internal temperature of the extruder within the rangewhere the volatiles can be removed. Specifically, the internaltemperature of the extruder is preferably about 190 to 260° C.Volatiles, as used herein, include unreacted monomers, diners and anunreacted. mercaptan.

[0045] For improving productivity, it is preferable to continuously feedthe reaction mixture to a devolatilization unit.

[0046] In this invention, the volatiles are purified preferably by amonomer purification process where a monomer containing a small amountof a mercaptan compound is purified using a catalyst containing at leastone element selected from the group of copper, cobalt, nickel andmanganese in the presence of molecular oxygen and of a compoundcontaining at least chlorine.

[0047] A molecular oxygen source which is present in the purificationprocess may be air, oxygen-rich air or oxygen.

[0048] The purification process may be in either liquid or gas phase.

[0049] For liquid phase purification, for example, a catalyst may beadded to a monomer containing a small amount of a mercaptan compound inthe presence of molecular oxygen and the mixture is, as necessary,stirred for a certain period. Molecular oxygen is preferably fed in anamount within the range of 0.1 to 50 mL/min per 100 mL of a reactionsolution. The amount of the metal compound as a catalyst is preferably0.01 to 1 parts by weight as a metal per one part by weight of themercaptan compound contained in the monomer as an impurity. Apurification temperature is preferably 0 to 80° C., more preferably 20to 60° C.

[0050] On the other hand for gas phase purification, a vaporized monomercontaining a small amount of mercaptan compound may be in contact with acatalyst under heating. The catalyst is usually used as a fixed bed, butmay be used as a moving or fluidized bed. A contact period is preferably0.1 to 10 sec. The monomer containing a mercaptan compound may bevaporized by, but not limited to, heating the monomer to its boilingpoint or higher at an ambient pressure or by vacuuming. The monomer canbe diluted with an inert gas such as nitrogen, argon and steam.

[0051] Molecular oxygen is fed to a 0.01 to 0.5 fold volume to thevolume of the vaporized solution to be purified, i.e., 0.01 to 0.5 molarratio to the monomer containing a mercaptan compound. Purification isusually conducted under the desired conditions of a temperature of 100to 200° C. and a pressure from several ten kPa (reduced pressure) toseveral hundred kPa (pressurized). The gaseous monomer after being incontact with the catalyst may be preferably trapped as a liquid by ausual process such as trapping it as a liquid under cooling or absorbingit with a solvent.

[0052] For the purification process, a catalyst containing at least oneelement selected from the group of copper, cobalt, nickel and manganese,may include compounds other than their chlorides when conductingpurification in a liquid phase; for example, carboxylates such asformates, acetates, citrates, oleates and naphthenates; inorganic acidsalts such as sulfates and nitrates; complexes such as acetylacetonates; oxides; or mixtures thereof. These compounds may becommercially available.

[0053] For gas phase purification, a catalyst may be an oxide containingany of the above metallic elements. Particularly preferable catalystsare represented by a general formula X_(a)Si_(b)Al_(c)O_(d) wherein Si,Al and O represent silicon, aluminum and oxygen, respectively; Xrepresents at least one element selected from the group of copper,cobalt, nickel and manganese; a, b, c and d represent atom ratios forindividual elements, provided that when a=1, b and c are 0 to 50 and dis an oxygen atom ratio required for satisfying atomic values of theabove individual components.

[0054] The compound represented by the general formulaX_(a)Si_(b)Al_(c)O_(d) may be prepared by, but not limited to, a priorwell known process where a catalyst precursor prepared by an appropriatemethod such as evaporation to dryness, precipitation and oxide mixingmethod is, as appropriate, formed into a desired shape by, e.g.,tabletting and then heating it, as long as it does not causesignificantly uneven distribution of components. Heating is preferablyconducted usually at 200 to 700° C. for a duration of 30 min or longer.Starting materials for preparing these catalysts may be an appropriatecombination of, for example, oxides, nitrides, carbonates, ammoniumsalts and hydroxides of individual elements.

[0055] A compound containing at least chlorine which is present duringpurification may be some form of chlorine-atom-containing compound suchas molecular chlorine, hydrochloric acid, sodium chloride, sodiumchlorate, calcium chloride, copper chloride, cobalt chloride, nickelchloride and manganese chloride. The compound may be fixed in thecatalyst during catalyst preparation or may be present in a liquid to bepurified or in a gas during purification. The amount of the compoundcontaining at least chlorine may be in a small amount to the catalystused, preferably 0.001 to 10 parts by weight as chlorine atom per 100parts by weight of the catalyst. When the amount of the compoundcontaining at least chlorine is less than 0.001 parts by weight, thereaction cannot be significantly promoted, while when more than 10 partsby weight is present, it may adversely affect a reaction unit and so on.

[0056] The monomer trapped as described above is distilled appropriatelyin the presence of a polymerization inhibitor such as hydroquinone andhydroquinone monomethyl ether to provide the high pure monomercontaining a reduced amount of impurities such as disulfide derived fromthe mercaptan compound. There are no limitations for distillationconditions, but distillation is preferably conducted by heating thecrude monomer to several ten ° C. under a reduced pressure.

[0057] When employing solution polymerization in this invention, asolvent is fed into a reactor in addition to a monomer and an initiatorIII. The solvent may be a known solvent such as toluene, xylenes,acetone, methyl ethyl ketone, methanol, ethanol, ethylbenzene, methylisobutyl ketone and n-butyl acetate; particularly preferably, methanol,methyl ethyl ketone, ethylbenzene and n-butyl acetate. The amount of thesolvent is preferably 40 wt % or less, more preferably 20 wt % or less,further preferably 10 wt % or less, to the total amount of the monomer,the initiator III, a chain transfer agent and the solvent.

[0058] The polymer content in the reaction mixture in the polymerizationzone is preferably 40 to 70 wt % for industrially advantageousproduction. The solvent is preferably recovered together with volatilesin a devolatilization step. Recovery can be conducted by, but notlimited to, supplying the reaction mixture to an appropriate apparatussuch as a vent-type extruder, whose internal temperature is preferablyabout 190 to 260° C.

[0059] A polymer prepared by the process of this invention may be usedto provide an optical fiber having improved optical properties.

[0060] There are no limitations for the structure of the optical fiber;specific examples are an SI type of optical fiber where a core and asheath are concentrically piled in whose interface a refractive indexabruptly changes, a GI type of optical fiber where a refractive indexcontinuously changes from the center to the periphery, and an opticalfiber where a refractive index changes stepwise from the center to theperiphery. Since a polymer prepared according to this invention exhibitsimproved optical properties, it is preferably used in a part throughwhich a light entering the optical fiber mainly passes, e.g, in acomponent constituting a core in an SI type of optical fiber.

[0061] For preparing an optical fiber, it is preferable to conductspinning using a multi-component spinning nozzle discharging a pluralityof materials to form a concentrically piled structure. Here, it ispreferable to feed a polymer from which volatiles have been removed inadvance, directly to the multi-component spinning nozzle for minimizingcontamination of the polymer with dusts and reducing thermal hysteresisof the polymer as much as possible. A multi-component spinning nozzlewith an at least two-layer structure may be used as appropriate. Forexample, a multi-component spinning nozzle having an at leastthree-layer structure is used for preparing an optical fiber where arefractive index changes stepwise from the center to the periphery. Forpreparing an SI type of optical fiber, spinning is conducted by feedinga core component and a sheath component to the inner and the outerlayers, respectively, of a two-layer type of multi-component spinningnozzle. A process for preparing an optical fiber is not limited to thatusing a multi-component spinning nozzle; for example, a core componentmay be first spun and a sheath component may be then melt-applied to theouter surface of the core for preparing an SI type of optical fiber.

[0062] A sheath component for preparing an SI type of optical fiber maybe, for example, a copolymer of vinylidene fluoride with a fluoroalkylvinyl ether, a methacrylate, an acrylate, tetrafluoroethylene,hexafluoropropene and vinyl acetate. A copolymer of a methacrylate oracrylate with a fluoroalkyl methacrylate or fluoroalkyl acrylate may bealso used. A polymer mainly comprising vinylidene fluoride ispreferable; specifically, a copolymer of vinylidene fluoride andtetrafluoroethylene containing 75 to 99 wt % of vinylidene fluoride, acopolymer consisting of 75 to 95 wt % of vinylidene fluoride, 4 to 20 wt% of tetrafluoroethylene and 1 to 10 wt % of hexafluoropropene, and acopolymer consisting of 75 to 95 wt % of vinylidene fluoride, 4 to 20 wt% of tetrafluoroethylene and 1 to 5 wt % of vinyl fluoride.

[0063] A plastic optical fiber has been rapidly used in short-rangeoptical transmission applications because it may be of a larger apertureand lighter and better in processability and workability than an opticalfiber with an inorganic glass as a core material. Its opticaltransparency has been, however, not yet satisfactory. In practice, aplastic optical fiber has been, therefore, used for at most several tenmeter optical transmission.

[0064] An optical transmission loss (a transmission loss) in the plasticoptical fiber is mainly due to a polymer as a core material;specifically, considerably due to optical absorbance and Rayleighscattering inherent in the polymer as well as optical absorption andscattering due to staining caused by impurities in the polymer orthermal hysteresis generated during preparation of the polymer. It is,therefore, a key for improving performance to prepare a plastic opticalfiber using an optically transparent polymer as a core material.

[0065] Optical fibers in which a particular polymer is used as a corematerial for improving transmission performance have been disclosed inJP-A 2-158702, JP-A 63-94203 and JP-A 63-95402. JP-A 2-158702 hasdisclosed a plastic optical fiber comprising a polymer with a weightaverage molecular weight of 80,000 to 200,000 consisting of ahomopolymer of methyl methacrylate and a copolymer of methylmethacrylate with another copolymerizable monomer as a core and apolymer with a lower refractive index than that of the core as a sheathcontaining butyl acetate up to 1000 ppm in the core.

[0066] However, butyl acetate in the optical fiber derives from butylacetate as a solvent in preparation of a polymer in solution polymer.Satisfactory performance cannot be achieved with an optical fibercontaining such a residual butyl acetate which increases a transmissionloss.

[0067] JP-A 63-94203 has disclosed a core-sheath type of plastic opticalfiber where a core component is a polymer comprising at least 80 wt % ofpolymethyl methacrylaie unit and a sheath component is a polymer havinga refractive index lower at least by 2% than the core component polymer,characterized in that a transmission loss for a light having awavelength of 400 nm is 400 dB/km or less and the amount of a polymerunit dimer in the core component polymer is 200 ppm or less; and hasdescribed that 200 ppm or less of the dimer allows a transmission lossto be reduced.

[0068] JP-A 63-95402has disclosed a core-sheath type of plastic opticalfiber where a core component is a polymer comprising at least 80 wt % ofpolymethyl methacrylate unit and a sheath component is a polymer havinga refractive index lower at least by 2% than the core component polymer,characterized in that between residual methyl methacrylate and residualmethyl methacrylate dimer there is a relationship represented by thefollowing equation:

300≧0.025×A+B   (V)

[0069] wherein A and B are the amounts of residual methyl methacrylateand residual methyl methacrylate dimer (ppm), respectively; and theamount of the residual methyl methacrylate contained in the corecomponent polymer is 4000 ppm or less to the polymer consisting ofmethyl methacrylate unit in the core component polymer; and hasdescribed that a plastic optical fiber having improved transmissionperformance (light transmission performance) can be achieved byadjusting the amounts of the residual methyl methacrylate monomer and ofthe residual methyl methacrylate dimer to proper levels.

[0070] All of these optical fibers described above, however, comprise asa core component a polymer prepared by polymerization in the presence ofa mercaptan chain transfer agent, leading to a significant problem thatoptical transmission performance in a plastic optical fiber is reduceddue to a sulfur atom contained in the chain transfer agent.

[0071] Thus, JP-A 2-43506 has disclosed a plastic optical fibercomprising a core made of a polymer from methyl methacrylate as a maincomponent and a sheath made of a polymer having a refractive index lowerthan that of the core, characterized in that the core is made of apolymer from methyl methacrylate as a main component, prepared bypolymerizing a monomer mainly containing methyl methacrylate in theabsence of a mercaptan chain transfer agent.

[0072] However, bulk polymerization in the absence of a mercaptan chaintransfer agent provides a polymer having an excessively higher molecularweight, and therefore it become difficult to provide a polymer havingimproved spinning processability. Specifically, optical distortion is soincreased during spinning that a plastic optical fiber having improvedtransmission performance cannot be provided. Therefore, polymerizationis practically conducted by solution polymerization using an inertsolvent. Use of an inert solvent may, however, lead to residual inertsolvent in the polymer of the core, which causes reduction intransmission performance. Furthermore, use of an inert solvent requiresa solvent separation/recovery step, resulting in a complexity of theprocess.

[0073] We have intensely investigated in an attempt to provide a plasticoptical fiber having improved transmission performance, and have finallyachieved a plastic optical fiber having considerably improvedtransmission performance by employing the following constitution.

[0074] This invention provides an optical fiber where a core materialcontains 200 ppm to 1000 ppm both inclusive of sulfur atoms bound to a(co)polymer while containing 5 ppm or less of sulfur atoms not bound tothe (co)polymer. In the optical fiber of this invention, the corepreferably contains 1 ppm or less of sulfur atoms not bound to the(co)polymer.

[0075] In the optical fiber of this invention, the (co)polymer in thecore preferably has a molecular terminal structure, which is representedby the following formula (VI) derived from a radical initiator:

[0076] wherein R is alkyl or fluoroalkyl.

[0077] In the optical fiber of this invention, the core materialpreferably comprises a homopolymer of methyl methacrylate or a copolymerof methyl methacrylate and other copolymerizable monomer.

[0078] In the optical fiber of this invention, the (co)polymer in thecore preferably has a molecular terminal structure, which is representedby the following formula (VII) derived from a radical initiator:

[0079] In the optical fiber of this invention, it is preferable that thecore material comprises two or more (co)polymer whose copolymercomposition and refractive index are mutually different, which areconcentrically piled such that refractive indices are sequentiallyreduced from the core center to the periphery. In a preferable opticalfiber, the core material is-selected from the group consisting of ahomopolymer of methyl methacrylate, a copolymer of methyl methacrylateand a fluoroalkyl methacrylate and a copolymer of methyl methacrylateand benzyl methacrylate.

[0080] The optical fiber of this invention is preferably prepared byassembling a plurality of islands, where each of the islands has a coreand the islands are separated from each other by other (co)polymer.

[0081] This invention also provides an optical fiber cable comprisingthe above optical fiber and a coating layer which is formed on the outersurface of the optical fiber.

[0082] This invention also provides an optical fiber cable with a plugcomprising the optical fiber cable and a plug being attached to an endof said optical fiber.

[0083] This invention also provides a process for manufacturing anoptical fiber comprising the steps of:

[0084] feeding a polymerization initiator, an alkyl mercaptan having 3to 6 carbon atoms and a monomer or a mixture of two or more monomersinto a reactor to form a reaction mixture containing a (co)polymer;

[0085] feeding the reaction mixture into a vent-type devolatilizationextruder by directly spraying the mixture to a screw in an inlet in thevent-type devolatilization extruder under a reduced pressure through asmall hole or slit for removing volatiles to provide a (co)polymer; and

[0086] forming an optical fiber using the (co)polymer as a corematerial,

[0087] where a feed rate of the reaction mixture to the vent-typedevolatilization extruder and screw diameter and screw revolution speedin the vent-type devolatilization extruder satisfy the followingrelationship of equation (9):

Q≦0.002×φ² ×{square root}{square root over (N)}  (9)

[0088] wherein Q is a feed rate of the reaction mixture (L/hr); φ is ascrew diameter (mm); and N is a screw revolution speed (rpm). In theabove manufacturing process, it is preferable that one of the monomersfed into the reactor is methyl methacrylate.

[0089] This invention also provides the above process for manufacturingan optical fiber in which the core material comprises a homopolymer ofmethyl methacrylate or a copolymer of methyl methacrylate and othercopolymerizable monomer, comprising the steps of:

[0090] feeding a polymerization initiator, an alkyl mercaptan having 3to 6 carbon atoms and methyl methacrylate monomer or a mixture of methylmethacrylate and other copolymerizable monomer into a reactor to producea reaction mixture containing a methyl methacrylate (co)polymer in 30 to60 wt %;

[0091] feeding the reaction mixture preheated to 170 to 205° C. andcompressed to a pressure equal to or higher than a vapor pressure ofmethyl methacrylate at the preheating temperature into a vent-typedevolatilization extruder for removing-volatiles to obtain a methylmethacrylate (co)polymer; and

[0092] forming an optical fiber using the (co)polymer as a corematerial,

[0093] where the reaction mixture is fed into a vent-typedevolatilization extruder by directly spraying the mixture to a screw inan inlet in the vent-type devolatilization extruder under a reducedpressure through a small hole or a narrow slit; and at least in the mostdownstream vent of the vent-type devolatilization extruder, atemperature and a pressure are 230 to 270° C. and 50 Torr or less,respectively.

[0094] This invention will be described in detail.

[0095] A polymer constituting the core in the optical fiber of thisinvention is preferably, but not limited to, a (co)polymer comprising a(meth)acrylate monomer, more preferably a homopolymer of methylmethacrylate monomer or a copolymer of methyl methacrylate and anothercopolymerizable monomer. Monomers copolymeriable with methylmethacrylate preferably include, but not limited to, various(meth)acrylate monomers such as fluoroalkyl methacrylates, benzylmethacrylate and methyl acrylate. A preferable fluoroalkyl methacrylateis 2,2,3,3-tetrafluoropropyl methacrylate because of its goodcopolymerizability with methyl methacrylate. For a methyl methacrylatecopolymer, it preferably contains 50 wt % or more, more preferably 60 wt% ore more, particularly preferably 80 wt % or more of methylmethacrylate unit.

[0096] A polymer constituting a core is generally prepared by heating,for a methyl methacrylate (co)polymer, methyl methacrylate monomer or amixture of methyl methacrylate monomer and another copolymerizablemonomer in a batch style or continuously for a certain period in thepresence of a radical polymerization initiator and a mercaptan chaintransfer agent for polymerization reaction; and then removing volatilessuch as unreacted monomers from the reaction mixture obtained. A properinert solvent may be, as appropriate, used to an adequately low levelnot to impede transmission performance, preferably 20 wt % or less, morepreferably 10 wt % or less.

[0097] A polymer is preferably prepared as follows. While reactants arepolymerized in a complete mixing type of reactor with substantiallyhomogenous stirring at a polymerization temperature of 110 to 160° C.and with an average residence time of 2 to 6 hrs to continuously producea reaction mixture, the polymer content of which is preferably 30 to 70wt %, more preferably 30 to 60 wt %.

[0098] There are no limitations for a devolatilization method and anyknown process may be employed. It is essential in this invention thatsulfur-containing components (as sulfur atoms) which are not bound tothe polymer are contained in the polymer in 5 ppm or less afterdevolatilization of the reaction mixture. For achieving this purpose, itis preferable to adjust a capacity of the devolatilization step and theamount of the reaction mixture fed to the devolatilization step.

[0099] Volatiles can be effectively removed, for example, using thevent-type extruder described in JP-B 52-17555. It is here preferablethat a reaction mixture preferably containing a polymer in 30 to 70 wt %is preheated to 170° C. or higher; the mixture is then directly sprayedto a screw in an inlet of the vent-type extruder through a narrowopening such as a small hole and a slit; most of volatiles are separatedand recovered in the first vent under a pressure of 500 Torr or lower;and then the residual volatiles are removed in the second ventdownstream of the first vent at 200 to 270° C., preferably 230 to 270°C. under a pressure of 50 Torr or lower. The residual volatiles may befurther removed using the third vent downstream of the above vents at230 to 270° C. under a pressure of 50 Torr or lower. Volatiles, as usedherein, include unreacted monomers, dimers and an unreacted mercaptan.

[0100] In the above process, more than 70 wt % of the polymer contentmay make the polymerization reaction difficult to conduct stably, whileless than 30 wt % increases duty for removing volatiles and thusprovides no industrial advantages.

[0101] A preheating temperature lower than 170° C. causes increase in acaloric value required for removing volatiles, so that it is difficultto produce a polymer having a composition according to this invention. Apreheating temperature higher than 205° C. is advantageous for removingvolatiles, but tends to cause formation/adhesion of colored materialsprobably due to sulfur-containing compounds in a preheater surface incontact with the liquid phase and the colored materials are entrained inthe polymer, leading to increase in a transmission loss. A preheatingtemperature is preferably 185 to 205° C.

[0102] When using a single screw vent-type extruder as a vent-typeextruder, it is preferable to select that the following relationship issatisfied between a feed rate of the reaction mixture and a size of thevent-type extruder, for ensuring that the content of thesulfur-containing compounds not bound to the polymer is 5 ppm or less:

Q≦0.002×φ² ×{square root}{square root over (N)}  (9)

[0103] wherein Q is a feed rate of the reaction mixture (L/hr); φ is ascrew diameter (mm); and N is a screw revolution speed (rpm).

[0104] In particular, when the condition is satisfied and a preheatingtemperature of the reaction mixture fed to the vent-type extruder is170° C. or higher, the sulfur-containing components can be significantlyeffectively removed.

[0105] A polymer in this invention can be prepared either in a batchstyle or continuously, as long as the content of the sulfur-containingcomponents not bound to the polymer can be 5 ppm or less in the polymerafter removing volatiles.

[0106] In preparation of a polymer in this invention, mercaptan chaintransfer agents for adjusting a polymer molecular weight is used foradjusting a viscosity in a melting step during shaping the polymer as anoptical fiber and for preventing increase in scattering elements due tostructure formation during shaping. Among the chain transfer agents,sulfur components which are bound to the polymer by the chain transferreaction do not increase an optical absorption loss when being heated ora scattering loss when being humidified, but rather improve its thermaldecomposition resistance. On the other hand, the residual mercaptan anddisulfide compound in the polymer which are not bound to the polymer maybe easily discolored by heating. Therefore, thermal hysteresis in aspinning step may easily cause discoloration and an absorption loss maybe increased particularly in a wavelength range of 600 nm or lower. Inan optical fiber prepared after spinning, the mercaptan and thedisulfide compound cause increase in an absorption loss at an elevatedtemperature and induce scattering loss in a higher humidity. Suchresidual mercaptan and disulfide compound significantly hamper theformation of an optical fiber having improved transmission performanceand the retention of optical transmission properties for a long time.

[0107] It is, therefore, necessary in this invention that a mercaptanchain transfer agent is used for controlling a polymer molecular weightwithin a proper range; the content of sulfur atoms which are bound tothe polymer in the core is 200 ppm to 1000 ppm both inclusive forimproving thermal decomposition resistance of the polymer; and thecontent of sulfur atoms which are not bound to the polymer is 5 ppm orless for preventing discoloration.

[0108] The content of sulfur atoms which are bound to the polymer ispreferably 400 ppm to 800 ppm both inclusive. If the content of sulfuratoms which are bound to the polymer is too low, the polymer has aninadequate thermal decomposition resistance, so that, for example, whenpreparing an optical fiber by melt spinning, the melt viscosity of thepolymer is too high to be difficult to conduct spinning. On the otherhand, if the content of sulfur atoms is too high, the melt viscosity istoo low to be difficult to conduct spinning.

[0109] Sulfur atoms which are not bound to a polymer, i.e., coloringmaterials such as an unreacted mercaptan and a disulfide compound formedby a reaction of the mercaptan, must be removed to be 5 ppm or less ofthe total content as sulfur atoms in the above devolatilization step.The content is preferably 3 ppm or less, more preferably 1 ppm or less,ideally an undetectable level.

[0110] Mercaptans which may be satisfactorily used in this inventioninclude alkyl mercaptans such as n-propyl, n-butyl, t-butyl, n-hexyl,n-octyl and n-dodecyl mercaptans. Mercaptans having a relatively highervapor pressure are preferably used because use of mercaptans having arelatively lower vapor pressure causes increase in duty in thedevolatilization step. In this regard, alkyl mercaptans having 3 to 6carbon atoms are preferable, including n-butyl and t-butyl mercaptans.Furthermore, n-butyl mercaptan is most preferable because a mercaptanhaving a large chain transfer constant can minimize its amount for use.

[0111] In the case of using a methacrylate polymer as a core in thisinvention, the polymer can exhibit further improved transmissionperformance when, besides the above condition for the sulfur-atomcontent, a molecular terminal structure derived from an initiator hasthe structure represented by formula (VI) mentioned below, particularly,by formula (VII) for a methyl methacrylate polymer. In the formulae, nis a natural number of 1 or more.

[0112] wherein R is alkyl or fluoroalkyl.

[0113] The molecular terminal structure derived from the radicalinitiator represented by formula (VII) indicates that the molecularterminal has the same structure as that of methyl methacrylate monomer.Thus, excellent transmission performance of methyl methacrylate can befully utilized without receiving influence of optical absorption oroptical scattering due to a different molecular structure derived froman initiator.

[0114] In the prior art, a terminal structure of a polymer derived froman initiator has not been studied because an initiator is used in anamount of only several ten ppm for preparation of a polymer. However,slight increase in a transmission loss may be critical in terms ofperformance in an application where an extreme transparency is requiredsuch as an optical fiber. We have pursued a material having quiteexcellent transmission performance; have intensely investigated focusingon a completely novel viewpoint, i.e., a terminal structure of thepolymer derived from the polymerization initiator; and thus haveachieved this invention.

[0115] An initiator which can provide a polymer having a terminalstructure represented by formula (VII) may be, for example, dimethyl2,2′-azobis(2-methylpropionate) (formula (III)).

[0116] Besides an initiator terminal having the same structure as methylmethacrylate structure, use of dimethyl 2,2′-azobis(2-methylpropionate)may provide an additional advantage. In general, an initiator isdecomposed to generate radicals for initiating polymerization reactionwhile part of the radical are recombined to form a stable compound whichdoes not contribute to the polymerization reaction. For dimethyl2,2′-azobis(2-methylpropionate), recombination of radicals generated bydecomposition mostly forms methyl methacrylate monomer which is used inthe present invention. Thus, a polymer with an extremely small amount ofimpurities other than the monomer can be prepared.

[0117] In this invention, transmission performance may be furtherimproved when a weight average molecular weight in a polymer whichconstitutes a core satisfies to be 70,000 to 100,000 both inclusive.

[0118] A weight average molecular weight of 70,000 to 100,000 bothinclusive is important for achieving good fluidity at a relatively lowertemperature during spinning. Specifically, an optical fiber ispreferably prepared by a process comprising separately feeding tomulti-component spinning nozzle a polymer for a core and aseparately-prepared polymer having a refractive index lower than that ofthe core polymer in fused forms for spinning. In the process, heatingthe polymer to an elevated temperature increases a transmission loss dueto discoloration caused by, for example, decomposition of components. Itis, therefore, required to fuse the materials at a temperature as low aspossible for a short heating duration. Even at a relatively lowertemperature, spinning with a higher melt viscosity may deteriorateoptical transmission performance due to a residual optical distortion.It is, therefore, necessary to ensure good fluidity at a relativelylower temperature.

[0119] It is, therefore, preferable that for a methyl methacrylatepolymer, a weight average molecular weight is 100,000 or less for meltspinning at a relatively lower temperature. A polymer with a molecularweight of 100,000 or less does not require heating to a very hightemperature, so that an optical absorption loss due to discoloration canbe minimized and optical distortion can be avoided because of itsrelatively lower melt viscosity, resulting in satisfactory transmissionperformance. A polymer with a weight average molecular weight of 70,000or more can exhibit good transmission performance and provide a durableoptical fiber because the polymer has an adequate mechanical strengthagainst, e.g., bending.

[0120] There are no limitations for the structure of the optical fiberof this invention; specific examples are an SI type of optical fiberwhere a core and a sheath are concentrically piled as a two-layerstructure in whose interface a refractive index abruptly changes, a GItype of optical fiber where a refractive index of a core continuouslychanges from its center to periphery, and a multi-layer optical fiberconsisting of a plurality of layers where a refractive index of a coredecreases stepwise from its center to periphery.

[0121] A multi-layer optical fiber preferably has a structure where acore consists of piled and non-mixed layers made of (co)polymers havingdifferent refractive indices. In the structure, between adjacent layersit is also possible to form a mixed layer of the (co)polymersconstituting the adjacent layers. In this invention, a part or all ofthe (co)polymer constituting the core is made of the polymer describedabove, preferably of a methyl methacrylate polymer. For the (co)polymersconstituting the core of the multi-layer optical fiber, it is preferableto use (co)polymers which are produced from the same monomer but havedifferent copolymer composition ratios as adjacent non-mixed layers,because a scattering loss can be minimized in the interface between thenon-mixed layers.

[0122] A GI type or multi-layer optical fiber can consist of either acore alone or a core and a sheath on the periphery of the core, which ismade of a polymer having a refractive index lower than that of theperiphery of the core. The sheath may consist of a plurality of layers.

[0123] An optical fiber of this invention may be a sea-island type wheremutually separated multiple islands are combined through a common sea.In the sea-island type optical fiber, an island may consist either of acore alone or of a core and a sheath. Each island may have a similarstructure to the above described multi-layer optical fiber. A diameterof each island is preferably 250 μm or less, more preferably 200 μm orless for minimizing leakage light out of the optical fiber (bend loss)when the optical fiber is bent. The sea-island type optical fiber can beused for multiplex communication by guiding different optical signals toindividual islands.

[0124] In this invention, a protective layer may be formed on the outersurface of an optical fiber of core-sheath structure or of a sea-islandtype optical fiber.

[0125] Materials which may be used for a sheath or protective layerinclude copolymers of vinylidene fluoride with a fluoroalkyl vinylether, a methacrylate, an acrylate, tetrafluoroethylene,hexafluoropropene and vinyl acetate. A copolymer of a methacrylate oracrylate with a fluoroalkyl methacrylate or fluoroalkyl acrylate may bealso used.

[0126] A polymer mainly comprising vinylidene fluoride or a fluoroalkylmethacrylate is preferable. Examples for a polymer mainly comprisingvinylidene fluoride include a copolymer of vinylidene fluoride andtetrafluoroethylene, which contains 75 to 99 wt % of vinylidenefluoride, a copolymer consisting of 75 to 95 wt % of vinylidenefluoride, 4 to 20 wt % of tetrafluoroethylene and 1 to 10 wt % ofhexafluoropropene, and a copolymer consisting of 75 to 95 wt % ofvinylidene fluoride, 4 to 20 wt % of tetrafluoroethylene and 1 to 5 wt %of vinyl fluoride. Examples for a polymer mainly comprising afluoroalkyl methacrylate include copolymers of a short-chain fluoroalkylmethacrylate, a long-chain fluoroalkyl methacrylate and methylmethacrylate (or methacrylic acid) and copolymers of methyl methacrylatewith a long-chain fluoroalkyl methacrylate or with methacrylic acid.

[0127] A sea material in a sea-island type optical fiber may be, forexample, selected from the polymers as described above for a sheath orprotective layer.

[0128] An optical fiber of this invention may be used as an opticalfiber cable by placing a coating layer on its periphery. The coatinglayer may be made of a conventionally-used material such as Nylon 12,polyvinyl chloride, poly(chlorotrifluoroethylene) copolymers,polyethylene, polyurethane and perprene.

[0129] The optical fiber may be used as an optical fiber cable with aplug by placing a plug on an end of an optical fiber cable. A well-knownplug may be used.

[0130] An optical fiber of this invention may be prepared by a knownprocess. For preparing an SI-, GI- or multi-layer type optical fiber,spinning is conducted preferably using a multi-component spinning nozzlewhich concentrically discharge a plurality of materials to form a piledstructure. A multi-component spinning nozzle with an at least two-layerstructure may be used as appropriate. For example, a multi-componentspinning nozzle with an at least three-layer structure is used forpreparing an optical fiber where a refractive index changes stepwisefrom the center to the periphery. For preparing an SI type of opticalfiber, spinning is conducted by feeding a core component and a sheathcomponent to the inner and the outer layers, respectively, of atwo-layer type of multi-component spinning nozzle. A process forpreparing an optical fiber is not limited to that using amulti-component spinning nozzle; for example, a core component may befirst spun and a sheath component may be then melt-applied to the outersurface of the core for preparing an SI type of optical fiber. Forpreparing a sea-island type optical fiber, a known multi-componentspinning nozzle may be preferably used for spinning.

[0131] This invention will be more specifically described with referenceto Examples.

[0132] Properties for a polymer which was used as a core material weredetermined as follows.

[0133] A) Determination of a Sulfur-Containing Component Content in aPolymer

[0134] i) Determination of a Content of Sulfur Atoms which are Bound toa Polymer

[0135] Determination was carried out using a Doman micro-coulometrictitrator MCTS-130. Specifically, a calibration curve was plotted by ameasurement for a standard sample whose sulfur-atom concentration wasknown. Then, a polymer used as a core material was dissolved in a10-fold volume of acetone and the solution was added dropwise tomethanol to precipitate the polymer. The polymer alone was separated andcollected, and dried to give a polymer sample. After measurement for thepolymer sample, a measured value was read from the calibration curve.The value was then converted into a value per a unit quantity of thepolymer to give a content of sulfur atoms which are bound to a polymer.

[0136] ii) Determination of a Content of Sulfur Atoms which are notBound to a Polymer

[0137] It was determined using a 5890 SERIES II gas chromatograph (HPCompany) with a TC-WAX column (G. L. Science Inc.) with a length of 30m, an inner diameter of 0.53 mm and a film thickness of 1.0 μm. A flamephotometric detector which is highly sensitive to sulfur was used toquantitatively analyze residual n-butyl mercaptan or n-octyl mercaptanin a polymer and a disulfide compound formed by reaction between thesetwo mercaptan molecules. This quantitative analysis was conducted byplotting a calibration curve by a measurement for a standard samplesolution in acetone whose sulfur concentration was known; conductingmeasurement for a sample solution in which a polymer is dissolved toabout 13 wt/vol %; and converting a value obtained from the calibrationcurve into a value for sulfur atoms to give a content of sulfur atomswhich are not bound to the polymer.

[0138] A sulfur-atom content was the total of sulfur-atom equivalentsfor n-butyl mercaptan and di-n-butyl disulfide when using n-butylmercaptan and for n-octyl mercaptan and di-n-octyl disulfide when usingn-octyl mercaptan.

[0139] B) Determination of a Molecular Weight by GPC

[0140] An HLC-8020 gas chromatograph (TOSOH Company) was used, which wasequipped with two GMHXL columns (TOSOH Company). A calibration curve wasplotted using THF as a solvent and a TSK standard polystyrene (TOSOHCompany). A sample was a 0.1 g/dL solution prepared by stilldissolution.

[0141] A weight average molecular weight Mw and a ratio Mw/Mn, whereinMw and Mn are a weight average molecular weight and a number averagemolecular weight, was determined with a commercially available GPC dataprocessor (TOSOH data processor SC-8010).

[0142] C) Repetitive Bending Test

[0143] Bending was repeated with a bending radius of 15 mm and an angleof 180° and a bending number until an optical fiber core was broken wasrecorded.

[0144] D) Determination of Residual Monomer and Residual Dimer Amounts

[0145] After preparing an optical fiber by spinning, only the corecomponent of the fiber was taken out to give a measurement sample, whichwas then subject to determination by a gas chromatography.

EXAMPLE 1

[0146] To a purified MMA were added dimethyl2,2′-azobis(2-methylpropionate) (Wako Pure Chemicals V-601, purity: 99wt %) in a ratio of 1.5×10⁻⁵ mol/1 mol monomer and n-butyl mercaptan(ELF ATOCHEM NORTH AMERICA INC, purity: 99.5 wt %) in a ratio of1.75×10⁻³ mol/1 mol monomer, respectively, and the mixture wascontinuously fed into a reactor in which a polymerization temperaturewas controlled to 135° C. and the mixture was stirred and mixed with astirring blade. In the polymerization, an average residence time of thereaction mixture in a polymerization zone was set to 4 hours.

[0147] After polymerization, the reaction mixture was continuously takenout from the reactor, and was continuously sent to a vented extruderwith a pump for separating and removing volatiles therefrom to obtain apolymer.

[0148] A polymer and a dimer contents in the reaction mixture were 44 wt% and 0.05 wt %, respectively, immediately after taking out it from thereactor. In the polymer obtained after removing volatiles from thereaction mixture, a residual monomer and a dimer contents were 0.1 wt %and 0.03 wt % or less, respectively. This polymer exhibited a weightaverage molecular weight (Mw) of 95,000 by a gel permeationchromatography (GPC) and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. Thus, thepolymer had a considerably narrow molecular weight distribution. Aheating loss temperature determined by a thermobalance was 286° C. and aglass transition point determined with a differential scanningcalorimeter was as high as 120° C., indicating that the polymer had goodthermal properties. During a continuous operation for 360 hours, theoperation was quite stable and in observation of the inside of thereactor after the operation, polymer adhesion to the reactor and foreignmaterial formation were not obserbed.

[0149] Subsequently, using a two-layer multi-component spinning nozzle,while the above polymer was fed as a core material to the inner layer ofthe two-layer multi-component spinning nozzle and separately prepared2,2,2-trifluoroethyl methacrylate polymer was fed as a sheath materialto the outer layer of the multi-component spinning nozzle, a meltmulti-component spinning was conducted at a multi-component spinningnozzle temperature of 210° C. to obtain an optical fiber having acore-sheath structure.

[0150] The optical fiber was evaluated for transmission performance.This optical fiber exhibited transmission losses of 70, 62 and 133 dB/kmat wavelengths of 520, 570 and 650 nm, respectively, indicating that theoptical fiber had significantly excellent optical properties with asmall transmission loss.

EXAMPLE 2

[0151] An optical fiber was prepared in a similar method as described inExample 1, except that a monomer mixture of 98 wt % of MMA and 2 wt % ofmethyl acrylate was used as a monomer, an initiator concentration was1.3×10⁻⁵ (mol)/monomer (mol) and a polymerization temperature was 150°C.

[0152] A polymer content in the reaction mixture was 52 wt %,immediately after taking out it from the reactor. In the polymerobtained after separating and removing volatiles from the reactionmixture, a residual monomer content in the polymer was 0.09 wt %.

[0153] The optical fiber exhibited transmission losses of 82, 78 and 138dB/km at wavelengths of 520, 570 and 650 nm, respectively, indicatingthat the optical fiber had significantly excellent optical properties.

EXAMPLES 3 to 8

[0154] Optical fibers were prepared and evaluated in a similar method asdescribed in Example 1. The reaction conditions are shown in Table 1together with those for Examples 1 and 2. Conditions other than thoseshown in Table 1 were the same as described in Example 1.

EXAMPLE 9

[0155] An optical fiber was prepared in a similar method as described inExample 1, excpt that n-octyl mercaptan was used instead of n-butylmercaptan. Transmission losses were 120, 87 and 135 dB/km at wavelengthsof 520, 570 and 650 nm, respectively. A transmission loss in a shortwavelength range was slightly higher due to insufficient removal of themercaptan while the initiator contributed to improvement in atransmission loss, especially in a long wavelength range. The resultsare shown in Table 1. TABLE 1 Monomer composition Polymerizationconditions Radical Chain transfer Polymerization Residence PolymerTransmission loss Exam. MMA* Comonomer initiator (mol/ agent** (mol/temperature time content at 570 nm No. (wt %) (wt %) monomer 1 mol)monomer 1 mol) (° C.) (hr) (wt %) (dB/km) 1 100 0 1.5 × 10⁻⁵ 1.75 × 10⁻³135 4 44 62 2 98 MA* 2 1.3 × 10⁻⁵ 1.75 × 10⁻³ 150 4 52 78 3 100 0 1.8 ×10⁻⁵ 1.70 × 10⁻³ 130 3.5 44 61 4 100 0 1.8 × 10⁻⁵  2.2 × 10⁻³ 125 4 4461 5 100 0 1.6 × 10⁻⁵ 1.75 × 10⁻³ 135 4 47 61 6 100 0 1.0 × 10⁻⁵  1.5 ×10⁻³ 130 3 35 72 7 98 EA* 2 3.2 × 10⁻⁵  2.0 × 10⁻³ 150 2 56 81 8 100 01.5 × 10⁻⁵  2.2 × 10⁻³ 120 5 45 64 9 100 0 1.6 × 10⁻⁵ 1.75 × 10⁻³ 135 447 87

COMPARATIVE EXAMPLE 1

[0156] An optical fiber was prepared in a similar method as described inExample 1, except that 2,2′-azobis(2,4,4-trimethylpentane) was used asan initiator and the amount of the initiator was 1.2×10⁻⁵ (mol)/MMA(mol).

[0157] A polymer content in the reaction mixture was 46 wt % immediatelyafter taking out it from the reactor. In the polymer obtained afterseparating and removing volatiles from the reaction mixture, a residualmonomer and a dimer contents were 0.1 wt % and 0.03 wt %, respectively.The optical fiber exhibited transmission losses of 80 and 140 dB/km atwavelengths of 570 and 650 nm, respectively, indicating that the opticalfiber had insufficient optical properties.

COMPARATIVE EXAMPLE 2

[0158] Polymerization was conducted for 48 hours under the monomercomposition and the polymerization conditions shown in Table 2 while theother conditions were similar as described in Example 1.

[0159] As seen from the table, a polymer content in a reaction mixture,however, considerably varied in a range of 42 wt % to 50 wt %immediately after taking out it from a reactor, and a polymerizationtemperature was also unstable, i.e., stable operation was difficult.When observing the inside of the reactor after operation, there wasfound a large amount of gelled polymer attachment on the reactor inside.

[0160] For an optical fiber prepared in a similar manner as described inExample 1 with the polymer thus obtained, a diameter was not uniformwith many locally thicker parts. Therefore, the level of the opticalfiber was insufficient for industrial use.

COMPARATIVE EXAMPLE 3

[0161] Polymerization was conducted for 24 hours under the monomercomposition and the polymerization conditions shown in Table 2 while theother conditions were similar as described in Example 1.

[0162] As seen from Table 2, a polymerization temperature was, however,unstable in a range of 130 to 140° C. and a polymer content in areaction mixture considerably varied in a range of 40 wt % to 55 wt %immediately after taking out it from a reactor, i.e., stable operationwas difficult. When observing the inside of the reactor after operation,there was found a large amount of gelled polymer attachment on thereactor inside.

[0163] For an optical fiber prepared in a similar manner as described inExample 1 with the polymer thus obtained, polymer lumps with a highermolecular weight (gel) were unevenly distributed, and a diameter was notuniform, i.e., there were alternately thicker and thinner partsTherefore, the level of the optical fiber was insufficient forindustrial use.

COMPARATIVE EXAMPLE 4

[0164] Polymerization was conducted for 100 hours under the monomercomposition and the polymerization conditions shown in Table 2 while theother conditions were similar as described in Example 1.

[0165] In the polymer obtained after separating and removing volatilesfrom the reaction mixture, a residual monomer and a dimer contents were0.1 wt % and 0.5 wt %, respectively, indicating that it was a polymerwith a considerably higher dimer content.

[0166] An optical fiber prepared in a similar manner as described inExample 1 using the polymer thus obtained exhibited transmission lossesof 140, 95 and 140 dB/km at wavelengths of 520, 570 and 650 nm,respectively. Thus, a transmission loss was particularly higher in ashort wavelength range.

COMPARATIVE EXAMPLE 5

[0167] Polymerization was conducted for 100 hours under the monomercomposition and the polymerization conditions shown in Table 2 while theother conditions were similar as described in Example 1.

[0168] In the polymer obtained after separating and removing volatilesfrom the reaction mixture, a residual monomer and a dimer contents were0.2 wt % and 0.6 wt %, respectively, indicating that it is a polymerwith a considerably higher dimer content. Furthermore, a polymer contentin a polymerization zone was low, i.e., productivity for the polymer waslow.

[0169] An optical fiber prepared in a similar manner as described inExample 1 using the polymer thus obtained exhibited transmission lossesof 142, 95 and 140 dB/km at wavelengths of 520, 570 and 650 nm,respectively. Thus, a transmission loss was particularly higher in ashort wavelength range. TABLE 2 Monomer composition Polymerizationconditions Radical Polymerization Residence Polymer Transmission lossComp. Exam. MMA* initiator Chain transfer agent** temperature timecontent at 570 nm No. (wt %) (mol/monomer 1 mol) (mol/monomer 1 mol) (°C.) (hr) (wt %) (dB/km) 1 100 1.2 × 10⁻⁵ 1.75 × 10⁻³ 135 4 46 80 2 1004.3 × 10⁻⁵ 1.80 × 10⁻³ 110 3 42*-50 — 3 100 8.7 × 10⁻⁵ 1.50 × 10⁻³130-140 1  40-55 — 4 100 8.7 × 10⁻⁵ 1.70 × 10⁻³ 165 5 49 95 5 100 2.2 ×10⁻⁵ 1.75 × 10⁻³ 150 5 25 95

EXAMPLE 10

[0170] To a purified MMA were added dimethyl2,2′-azobis(2-methylpropionate) (Wako Pure Chemicals V-601, purity: 99wt %) in a ratio of 1.8×10⁻⁵ mol/1 mol monomer and n-butyl mercaptan(ELF ATOCHEM NORTH AMERICA INC, purity: 99.5 wt %) in a ratio of1.8×10⁻³ mol/1 mol monomer, respectively, and the mixture wascontinuously fed into a complete-mixing type reactor in which apolymerization temperature was controlled to 130° C. and the mixture wasstirred and mixed with a stirring blade. In the polymerization, anaverage residence time of the reaction mixture in a polymerization zonewas set to 3.6 hours.

[0171] While the reaction mixture was continuously taken out from thereactor, the mixture heated to 190° C. was continuously sent to arear-vented type of 3-vent single-screw devolatilization extruder havinga screw diameter of 40 mm using a pump at a rate of 15 L/hr forseparating and removing volatiles to obtain a polymer. A pressure in aninlet (rear vent: the first vent) was 100 Torr, while pressures in thesecond and the third vents were 50 Torr. An extruder temperature in theinlet was set to 220° C. while those in the second and the third ventsto 240° C. A screw revolution speed was 60 rpm. A polymer contentimmediately after taking out the mixture from the reactor was 45 wt %,which was calculated from the amount of the reaction mixture fed and theamount of the polymer collected after removing the volatiles. Thepolymer extruded without being exposed to the air from the tip of theextruder was continuously and directly fed to a two-layermulti-component spinning nozzle. While the above polymer was fed as acore material to the inner layer of the two-layer multi-componentspinning nozzle and a separately prepared polymer of 51 wt parts of2,2,2-trifluoroethyl methacrylate, 30 wt parts of1,1,2,2-tetrahydroperfluorodecyl methacrylate, 18 wt parts of methylmethacrylate and 1 wt part of methacrylic acid was fed as a sheathmaterial to the outer layer of the nozzle, a melt multi-componentspinning was conducted under a constant nozzle pressure of 30 kg/cm² andat a nozzle temperature of 220° C. to obtain an optical fiber having acore-sheath structure whose fiber diameter was 1000 μm.

[0172] The optical fiber thus obtained was evaluated for transmissionperformance and residual volatiles in the core material.

[0173] Only the polymer constituting the core of the optical fiber wastaken out to be subject to measurement. In the polymer, a residualmonomer and a dimer contents were 0.24 wt % and 0.05 wt %, respectively.

[0174] In this polymer, a content of sulfur components (as sulfur atoms)which are bound to the polymer (a bound-sulfur content) was 600 ppm,while a content of sulfur components (as sulfur atoms) which are notbound to the polymer (a residual-sulfur content) was 0.7 ppm.

[0175] This polymer exhibited a weight average molecular weight (Mw) of90,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 120° C., indicating that the polymer had good thermalproperties.

[0176] This optical fiber exhibited low transmission losses of 70, 62and 125 dB/km at wavelengths of 520, 570 and 650 nm, respectively, i.e.,it had significantly excellent optical properties.

[0177] The optical fiber after coated with polyethylene to an outerdiameter of 2.2 mm was subject to a repetitive bending test, Itexhibited good mechanical strength with a bending number of 20,000.

EXAMPLE 11

[0178] A polymer was prepared in a similar method as described inExample 10, except that a concentration of n-butyl mercaptan was2.0×10⁻³ mol/1 mol monomer. Subsequently, an optical fiber with a fiberdiameter of 1000 μm was prepared in a similar manner as described inExample 10, except that a nozzle temperature was set to 210° C.

[0179] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.20 wt % and 0.06 wt %, respectively.

[0180] In this polymer, a content of sulfur components which are boundto the polymer was 670 ppm, while a content of sulfur components whichare not bound to the polymer was 1.0 ppm.

[0181] This polymer exhibited a weight average molecular weight (Mw) of80,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 120° C., indicating that the polymer had good thermalproperties.

[0182] This optical fiber exhibited low transmission losses of 68, 60and 121 dB/km at wavelengths of 520, 570 and 650 nm, respectively, i.e.,it had significantly excellent optical properties. In a repetitivebending test carried out in a similar mannner as described in Example10, it exhibited good mechanical strength with a bending number of15,000.

EXAMPLE 12

[0183] A polymer was prepared in a similar method as described inExample 10, except that the amount of dimethyl2,2′-azobis(2-methylpropionate) was 2.0×10⁻⁵ mol/1 mol monomer, theamount of n-butyl mercaptan was 2.0×10⁻³ mol/1 mol monomer, apolymerization temperature was 126° C. and an average residence time was3.0 hrs. Subsequently, an optical fiber having a fiber diameter of 1000μm was obtained in a similar manner as described in Example 10, exceptthat a nozzle temperature was set to 210° C.

[0184] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.16 wt % and 0.018 wt %, respectively.

[0185] In this polymer, a content of sulfur components which are boundto the polymer was 640 ppm, while a content of sulfur components whichare not bound to the polymer was 0.9 ppm.

[0186] This polymer exhibited a weight average molecular weight (Mw) of82,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 120° C., indicating that the polymer had good thermalproperties.

[0187] This optical fiber exhibited low transmission losses of 71, 62and 124 dB/km at wavelengths of 520, 570 and 650 nm, respectively, i.e.,it had significantly excellent optical properties. In a repetitivebending test, it exhibited good mechanical strength with a bendingnumber of 15,000 equivalent to that in Example 11.

[0188] The polymer in this example had properties almost equivalent tothose for the polymer in Example 11, except for a lower dimer content.In this example, despite lower monomer and dimer contents in thepolymer, a transmission loss was substantially equivalent to that inExample 11.

EXAMPLE 13

[0189] A polymer was prepared in a similar method as described inExample 10, except that an initiator was2,2′-azobis(2,4,4-trimethylpentane) in a ratio of 1.3×10⁻⁵ mol/1 molmonomer. A polymer content immediately after taking out the reactionmixture from the reactor, which was calculated from the amount of thereaction mixture fed and the amount of the polymer produced afterremoving the volatiles, was 45 wt % equivalent to the value in Example10. Subsequently, an optical fiber having a fiber diameter of 1000 μmwas prepared in a similar manner as described in Example 10.

[0190] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.23 wt % and 0.05 wt %, respectively.

[0191] In this polymer, a content of sulfur components which are boundto the polymer was 610 ppm, while a content of sulfur components whichare not bound to the polymer was 0.7 ppm.

[0192] This polymer exhibited a weight average molecular weight (Mw) of90,000 by GPC me3thod and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 120° C., indicating that the polymer had good thermalproperties.

[0193] This optical fiber exhibited transmission losses of 82, 72 and130 dB/km at wavelengths of 520, 570 and 650 nm, respectively. In arepetitive bending test, it exhibited good mechanical strength with abending number of 20,000 equivalent to that in Example 10.

[0194] The process of this example was almost as similar as described inExample 10, except that a different initiator was used. In this example,a transmission loss was slightly higher, despite that the residualmonomer content, the dimer content, the molecular weight and the heathistory were almost as similar as described in Example 10. The opticalfiber, however, had better transmission performance than that preparedby a conventional manufacturing process, because of a lower content ofsulfurs which are not bound to the polymer in the polymer.

EXAMPLE 14

[0195] A polymer was prepared in a similar method as described inExample 10, except that a concentration of n-butyl mercaptan was1.4×10⁻³ mol/1 mol monomer. Subsequently, an optical fiber having afiber diameter of 1000 μm was obtained in a similar manner as describedin Example 10, except that a nozzle temperature was set to 235° C.

[0196] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.24 wt % and 0.06 wt %, respectively.

[0197] In this polymer, a content of sulfur components which are boundto the polymer was 490 ppm, while a content of sulfur components whichare not bound to the polymer was 1.2 ppm.

[0198] This polymer exhibited a weight average molecular weight (Mw) of110,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter was120° C.

[0199] This optical fiber exhibited transmission losses of 81, 70 and132 dB/km at wavelengths of 520, 570 and 650 nm, respectively. The fiberexhibited a higher transmission loss than that in Example 10, but itstransmission performance was better than an optical fiber prepared by aconventional manufacturing process. In a repetitive bending test, itexhibited good mechanical strength with a bending number of 22,000.

Example 15

[0200] A polymer was prepared in a similar method as described inExample 10, except that a concentration of n-butyl mercaptan was2.5×10⁻³ mol/1 mol monomer. Subsequently, an optical fiber having afiber diameter of 1000 μm was obtained in a similar manner as describedin Example 10, except that a nozzle temperature was set to 205° C.

[0201] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.17 wt % and 0.02 wt %, respectively.

[0202] In this polymer, a content of sulfur components which are boundto the polymer was 720 ppm, while a content of sulfur components whichare not bound to the polymer was 1.0 ppm.

[0203] This polymer exhibited a weight average molecular weight (Mw) of64,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 117° C., indicating that the polymer had good thermalproperties.

[0204] Although exhibiting relatively lower mechanical strength with abending number of 8,000 in a repetitive bending test, the optical fiberexhibited low transmission losses of 68, 60 and 120 dB/km at wavelengthsof 520, 570 and 650 nm, respectively, i.e., it had significantlyexcellent optical properties.

COMPARATIVE EXAMPLE 6

[0205] A polymer was prepared in a similar method as described inExample 10, except that in a ratio of 1.8×10⁻³ mol/1 mol monomer ofn-octyl mercaptan was used instead of n-butyl mercaptan. Subsequently,an optical fiber having a fiber diameter of 1000 μm was obtained in asimilar manner as described in Example 10.

[0206] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.24 wt % and 0.06 wt %, respectively.

[0207] In this polymer, a content of sulfur components which are boundto the polymer was 590 ppm, while a content of sulfur components whichare not bound to the polymer was 27 ppm.

[0208] This polymer exhibited a weight average molecular weight (Mw) of90,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 120° C., indicating that the polymer was good in the sense ofthermal properties.

[0209] This optical fiber exhibited transmission losses of 92, 85 and136 dB/km at wavelengths of 520, 570 and 650 nm, respectively. Thus, ithad a higher transmission loss despite that the residual monomercontent, the dimer content, the molecular weight and the heat historywere almost similar as described in Example 10.

COMPARATIVE EXAMPLE 7

[0210] A polymer was prepared in a similar method as described inExample 10, except that for operation conditions in a devolatilizationstep, a reaction mixture at the polymerization temperature of 130° C.was directly fed to an extruder without further heating, and a pressureof the inlet (rear vent) was 600 Torr while pressures of the second andthe third vents were 50 Torr for devolatilization. Subsequently, anoptical fiber having a fiber diameter of 1000 μm was obtained in asimilar manner as described in Example 10.

[0211] In a similar manner as described in Example 10, only the polymerconstituting the core of the optical fiber was taken out to be subjectto measurement. In the polymer, a residual monomer and a dimer contentswere 0.41 wt % and 0.09 wt %, respectively.

[0212] In this polymer, a content of sulfur components which are boundto the polymer was 600 ppm, while a content of sulfur components whichare not bound to the polymer was 8 ppm.

[0213] This polymer exhibited a weight average molecular weight (Mw) of90,000 by GPC method and a ratio of weight average molecularweight/number average molecular weight (Mw/Mn) was 2.0. A glasstransition point determined with a differential scanning calorimeter wasas high as 117° C., indicating that the polymer was good in the sense ofthermal properties.

[0214] The optical fiber exhibited higher transmission losses of 90, 83and 135 dB/km at wavelengths of 520, 570 and 650 nm, respectively.

[0215] The above results are summarized in Table 3. TABLE 3 BoundResidual Residual Residual Molecular sulfur sulfur monomer dimerTransmission loss Exam. Radical weight content content content content(dB/km) Bending No. initiator (Mw) (ppm) (ppm) (ppm) (ppm) 520 nm 570 nm650 nm number Ex. 10 (A) 90,000 600 0.7 2400 500 70 62 125 20,000 Ex. 11(A) 80,000 670 1.0 2000 600 68 60 121 15,000 Ex. 12 (A) 82,000 640 0.91600 180 71 62 124 15,000 Ex. 13 (B) 90,000 610 0.7 2300 500 82 72 13020,000 Ex. 14 (A) 110,000 490 1.2 2400 600 81 70 132 22,000 Ex. 15 (A)64,000 720 1.0 1700 200 68 60 120  8,000 Comp. Ex. 6 (A) 90,000 590 272400 600 92 85 136 20,000 Comp. Ex. 7 (A) 90,000 600 8 4100 900 90 83135 18,000

EXAMPLE 16

[0216] Preparation solution 1 which was prepared by adding, to an MMA,dimethyl 2,2′-azobis(2-methylpropionate) (Wako Pure Chemicals V-601,purity: 99 wt %) in a ratio of 1.8×10⁻⁵ mol/1 mol monomer and n-butylmercaptan (ELF ATOCHEM NORTH AMERICA INC, purity: 99.5 wt %) in a ratioof 1.8×10⁻³ mol/1 mol monomer, respectively, was continuously fed into areactor 1. Separately, preparation solution 2 which was prepared byadding, to a mixture of MMA and 2,2,3,3-tetrafluoropropyl methacrylate(4FM) (80/20 wt %), dimethyl 2,2′-azobis(2-methylpropionate) (Wako PureChemicals V-601, purity: 99 wt %) in a ratio of 1.8×10⁻⁵ mol/1 molmonomer and n-butyl mercaptan (ELF ATOCHEM NORTH AMERICA INC, purity:99.5 wt %) in a ratio of 1.8×10⁻³ mol/1 mol monomer, was continuouslyfed into a reactor 2. In both reactors 1 and 2, a polymerizationtemperature was controlled to 130° C. and the mixture was stirred andmixed with a stirring blade. In polymerization, an average residencetime of each reaction mixture in a polymerization zone was set to 3.6hours. While these reaction mixtures were continuously taken out fromthe reactors 1 and 2, the mixtures heated to 190° C. were continuouslysent to rear-vented type of 3-vent single-screw devolatilizationextruders 1 and 2 each having a screw diameter of 40 mm using a pump ata rate of 15 L/hr for separating and removing volatiles to obtainpolymers. A pressure in an inlet (rear vent: the first vent) was 100Torr, while pressures in the second and the third vents were 50 Torr. Anextruder temperature in the inlet was set to 220° C. while those in thesecond and the third vents to 240° C. A screw revolution speed was 60rpm.

[0217] Polymer contents immediately after taking out the mixtures fromthe reactors 1 and 2 were 45 wt % and 47 wt %, respectively, which werecalculated from the amount of the reaction mixture fed and the amount ofthe polymer collected after removing the volatiles.

[0218] A residual monomer content in the polymer 1 obtained from thedevolatilization extruder 1 was 0.24 wt % for MMA, while residualmonomer contents in the polymer 2 obtained from the devolatilizationextruder 2 were 0.14 wt % for MMA and 0.11 wt % for 4FM. Contents ofsulfur components which are chemically bound to the polymers 1 and 2were 600 and 560 ppm, respectively, while contents of sulfur componentswhich are not chemically bound to the polymers were 0.7 and 1 ppm,respectively.

[0219] Then, the polymers 1 and 2 extruded without being exposed to theair from the tips of the devolatilization extruders 1 and 2 werecontinuously and directly fed to a three-layer multi-component spinningnozzle. While the above polymers 1 and 2 were fed as cores 1 (the innerlayer of the core) and 2 (the outer layer of the core) and a separatelyprepared polymer of 28 wt parts of 1,1,2,2-tetrahydroperfluorodecylmethacrylate (17FM), 71 wt parts of MMA and 1 wt part of methacrylicacid (MAA) was fed as a sheath material, a melt multi-component spinningwas conducted to obtain a multi-layered optical fiber having a fiberdiameter of 750 μm, whose transmission performance was evaluated. Thecore 1 had a diameter of 450 μm while the core 2 had a thickness of 135μm and a sheath thickness of 15 μm. In this process, the multi-componentspinning nozzle conditions were controlled as a constant nozzle pressureof 30 kg/cm² and a nozzle temperature of 220° C.

[0220] This optical fiber exhibited extremely low transmission losses of70, 62 and 119 dB/km at wavelengths of 520, 570 and 650 nm,respectively, i.e., it had significantly excellent optical properties.

[0221] A transmission band for a fiber length of 50 m was 550 MHz.

[0222] In a durability test at temperatures of 85° C. and 70° C. and ata relative humidity of 95 % for 10,000 hrs, a transmission lossincreased by 30 dB/km or less and a transmission band was notsignificantly changed.

EXAMPLE 17

[0223] Four different core materials were prepared in a similar methodas described in Example 16, except that preparation conditions were setas shown in Table A, and these core materials and sheath andprotective-layer materials shown in Table A were fed to a 6-layermulti-component spinning nozzle to obtain an optical fiber shown inTable A. The evaluation results are shown in Table B.

EXAMPLE 18

[0224] Three different core materials were prepared in a similar methodas described in Example 16, except that the preparation conditions wereset as shown in Table A, and these core materials and a sea material anda protective-layer material shown in Table A were fed to amulti-component spinning nozzle for a multiple core fiber to obtain asea-island type optical fiber as shown in Table A where 37 islandsconsisting of three kinds of concentrically piled core materials wereassembled through a sea part and a protective layer was formed on theperiphery. On the periphery of the optical fiber was applied a coatinglayer made of a blend of vinyl chloride and an ethylene/vinyl acetatecopolymer (Toyo Ink Co. Ltd.: 314) to obtain a multi-core optical fibercable having an outer diameter φ of 2.2 mm. There were observed nodamages in the core part of the periphery. The evaluation results areshown in Table B. The optical fiber was cut to give a 5 m piece, whoseends were ground. Introducing light from its one end, brightness foreach island was observed at the other end, and brightness wassubstantially even throughout the fiber.

EXAMPLES 19 and 20

[0225] Each of optical fibers shown in Table A was prepared in a similarmethod as described in Example 16, except that the preparationconditions were set as shown in Table A. The evaluation results areshown in Table B.

COMPARATIVE EXAMPLE 8

[0226] An optical fiber shown in Table A was prepared in a similarmethod as described in Example 16, except that the preparationconditions were set as shown in Table A.

[0227] In both reactors 1 and 2, a polymer content in a reactionmixture, however, considerably varied in a range of 42 wt % to 50 wt %,and a polymerization temperature was also unstable, i.e., stableoperation was difficult. When observing the insides of the reactors 1and 2 after operation, there was found a large amount of gelled polymerattachment on the reactor inside. For an optical fiber prepared in asimilar manner as described in Example 16 with the polymer thusobtained, a diameter of the fiber was not uniform with many locally:thicker parts. Therefore, the level of the optical fiber wasinsufficient for industrial use.

COMPARATIVE EXAMPLE 9

[0228] An optical fiber shown in Table A was prepared in a similarmethod as described in Example 16, except that the preparationconditions were set as shown in Table A.

[0229] Although in both reactors 1 and 2 a polymerization temperaturewas intended to be adjusted to 130° C. by controlling a jackettemperature in the reactors, it was not stable in a range of 130 to 140°C., and a polymer content in a reaction mixture considerably varied in arange of 40 wt % to 55 wt % immediately after taking out it from thereactor, i.e., stable operation was difficult. When observing theinsides of the reactors 1 and 2 after operation, there was found a largeamount of gelled polymer attachment on the reactor inside.

[0230] For an optical fiber prepared in a similar manner as described inExample 16 with the polymer thus obtained, polymer lumps with a highermolecular weight (gel) were unevenly distributed, and a diameter of thefiber was not uniform, i.e., there were alternately thicker and thinnerparts. Therefore, the level of the optical fiber was insufficient forindustrial use.

EXAMPLE 21

[0231] Polymerization was conducted in a similar method as described inExample 10. A reaction mixture was continuously taken out from areactor, transferred while being heated to 190° C., and fed to asingle-screw devolatilization extruder as described in Example 10 forseparating and removing volatiles to obtain a polymer. The operationconditions for the single-screw devolatilization extruder were similaras described in Example 10.

[0232] Subsequently, using the polymer extruded from the tip of theextruder in a similar manner as described in Example 10, a core-sheathtype optical fiber having a fiber diameter of 1000 μm was obtained,which was evaluated for its transmission performance and residualvolatiles in the core.

[0233] In the core, a residual monomer and a dimer contents were 0.78and 0.12 wt %, respectively. A content of sulfur components which arebound to the polymer in the core was 600 ppm, while a content of sulfurcomponents which are not bound to the polymer was 8.9 ppm.

[0234] A weight average molecular weight (Mw) by GPC method was 90,000and a ratio of Mw/Mn was 2.0, which was equivalent to the value inExample 10.

[0235] The optical fiber exhibited large transmission losses of 93, 87and 136 dB/km at 520, 570 and 650 nm, respectively.

EXAMPLE 22

[0236] In the process described in Example 1, the volatiles separatedand removed in the devolatilization step were cooled in a condenser tocollect a liquid (1000 kg) in a tank. For the collected liquid, n-butylmercaptan was quantitatively analyzed by hydrogen flame gaschromatography. The result was 1,500 ppm. The collected liquid appearedto be very slight pale yellow. To 1000 kg of the collected liquid wereadded 250 g of cupric oxide (copper (II) oxide) as a catalyst and 10 gof cupric chloride (copper (II) chloride) as a chloride. The mixturekept at 60° C. was stirred with a double propeller-blade stirrer at 200rpm while feeding air from the bottom of the tank in a rate of 10 L/min.After 4 hours, the liquid was cooled to 20° C. and quantitativelyanalyzed for an amount of residual n-butyl mercaptan. The result wasbelow a determination limit (1 ppm).

[0237] After filtration, to the liquid was added hydroquinone as apolymerization inhibitor to 50 ppm, and the mixture was distilled at 40°C. under 100 Torr for purification. For a distillate after about 98%distillation, n-butyl mercaptan and di-n-butyl disulfide werequantitatively analyzed. The results were below a determination limit (1ppm).

[0238] Feeding the distillate as a starting material into the reactor inExample 1, a polymer was prepared under the conditions similar asdescribed in Example 1 and a core-sheath type plastic optical fiber wasprepared in a similar manner as described in Example 1.

[0239] The optical fiber exhibited transmission losses of 71, 62 and 132dB/km at 520, 570 and 650 nm, respectively, which were equivalent tothose in Example 1, indicating that the fiber had a lower transmissionloss and good optical properties.

EXAMPLE 23

[0240] To a reaction raw material consisting of a monomer mixture of thedistillate in Example 22 and methyl acrylate (MA) (98:2 by weight) wereadded dimethyl 2,2′-azobis(2-methylpropionate) (Wako Pure ChemicalsV-601, purity: 99 wt %) in a ratio of 1.5×10⁻⁵ mol/1 mol monomer andn-octyl mercaptan (ELF ATOCHEM NORTH AMERICA INC, purity: 99.5 wt %) ina ratio of 1.75×10⁻³ mol/1 mol monomer, and the mixture was continuouslyfed into a reactor for polymerization under the conditions of apolymerization temperature of 135° C. and an average residence time of 4hours to conduct polymerization in a similar manner as described inExample 1. Then, the mixture was fed to a vented extruder for separatingand removing volatiles to obtain a polymer pellet. With respect to thereaction mixture immediately after taking out it from the reactor, apolymer and a dimer contents were 44 wt % and 0.05 wt %, respectively.With respect to the polymer obtained after removing the volatiles fromthe reaction mixture, a residual monomer and a dimer contents were 0.1wt % and 0.03 wt %, respectively.

[0241] This polymer exhibited a weight average molecular weight (Mw) of95,000 by gel permeation chromatography (GPC method) and a ratio ofweight average molecular weight/number average molecular weight (Mw/Mn)was 2.0. Thus, the polymer had a considerably narrow molecular weightdistribution.

[0242] A heating loss temperature determined with a thermobalance was295° C. and a glass transition point determined with a differentialscanning calorimeter was as high as 117° C., indicating that the polymerhad good thermal properties. With respect to the polymer, n-butylmercaptan was quantitatively analyzed, but not detected.

[0243] The polymer was fed as a starting material to a material inlet inan injection molding machine for molding under the conditions of acylinder temperature of 250° C. and a molding cycle of 30 sec, toprovide 100 plates having dimensions of 110 mm×110 mm×5 mm. During themolding process, bad smell was not observed and there were no problemsin terms of a work environment. The plates were macroscopically observedfor their coloring and coloring was not observed in comparison with acommercially available methacrylic resin molding material (MitsubishiRayon Co. Ltd.: Acrypet VH).

[0244] As described above, a methyl methacrylate polymer havingadequately good optical properties and a plastic optical fiber havingimproved transmission performance can be prepared according to thisinvention.

[0245] This invention can also provide an optical fiber, an opticalfiber cable and an optical fiber cable with a plug having extremelylower transmission loss, which cannot be achieved according to the priorart. Furthermore, this invention can provide a process for readilymanufacturing such an optical fiber. TABLE A Example 16 Example 17Example 18 Example 19 Core number 1 1 37 1 Core 1 Composition MMA MMAMMA MMA Initiator V601 V601 V601 di-tert-butyl peroxide Initiator conc.(*10⁻⁵ 1.8 1.8 1.8 1.8 mol/1 mol monomer) Polymerization temp. 130 130130 155 (° C.) Mercaptan, conc. (*10⁻³ n-BtSH n-BtSH n-BtSH n-BtSH mol/1mol monomer) 1.8 1.8 1.8 1.8 Average residence time 3.6 3.6 3.6 3.6(hrs) Polymer content (wt %) 45 45 45 45 Residual monomer (MMA, 0.240.18 0.18 0.11 wt %) Residual monomer (4FM, wt %) Bound sulfur component600 600 600 620 (ppm) Non-bound sulfur com- 0.7 0.8 0.8 2 ponent (ppm)Diameter (μm) 450 400 50% (Note 1) 450 Core 2 Composition MMA/4FM =MMA/4FM = MMA/4FM = MMA/4FM = 80/20 wt % 90/10 wt % 90/10 wt % 80/20 wt% Initiator V601 V601 V601 di-tert-butyl peroxide Initiator conc. (*10⁻⁵1.8 1.8 1.8 1.8 mol/1 mol monomer) Polymerization temp. 130 130 130 155(° C.) Mercaptan, conc. (*10⁻³ n-BtSH n-BtSH n-BtSH n-BtSH mol/1 molmonomer) 1.8 1.8 1.8 1.8 Average residence time 3.6 3.6 3.6 3.6 (hrs)Polymer content (wt %) 47 47 47 47 Residual monomer (MMA, 0.14 0.19 0.190.15 wt %) Residual monomer (4FM, 0.11 0.02 0.02 0.13 wt %) Bound sulfurcomponent 560 580 580 605 (ppm) Non-bound sulfur com- 1 0.9 0.9 2 ponent(ppm) Thickness (μm) 135 75 20% (Note 1) 135 Core 3 Composition —MMA/4FM = MMA/4FM = — 80/20 wt % 80/20 wt % Initiator — V601 V601 —Initiator conc. (*10⁻⁵ — n-BtSH n-BtSH — mol/1 mol monomer) 1.8 1.8Polymerization temp. — 130 130 — (° C.) Mercaptan, conc. (*10⁻³ — 1.81.8 — mol/1 mol monomer) Average residence time — 3.6 3.6 — (hrs)Polymer content (wt %) — 47 47 — Residual monomer (MMA, — 0.14 0.14 — wt%) Residual monomer (4FM, — 0.07 0.07 — wt %) Bound sulfur component —560 560 — (ppm) Non-bound sulfur com- — 1 1 — ponent (ppm) Thickness(μm) — 50 10% (Note 1) — Core 4 Composition — MMA/4FM = — — 70/30 wt %Initiator — V601 — — Initiator conc. (*10⁻³ — n-BtSH — — mol/1 molmonomer) 1.8 Polymerization temp. — 130 — — (° C.) Mercaptan, conc.(*10⁻³ — 1.8 — — mol/1 mol monomer) Average residence time — 3.6 — —(hrs) Polymer content (wt %) — 48 — — Residual monomer (MMA, — 0.09 — —wt %) Residual monomer (4FM, — 0.1 — — wt %) Bound sulfur component —550 — — (ppm) Non-bound sulfur com- — 1 — — ponent (ppm) Thickness (μm)— 30 — — Sheath Copolymer composition 17FM/MMA/MAA = 17FM/MMA/MAA =(VdF/TFE) = 17FM/MMA/MAA = 28/71/1 wt % 30/69/1 wt % 80/20 wt % 28/71/1wt % MI = 40 (sea material) Thickness (μm) 15 10 15 % (Note 1)(Sea) 15Protect. Copolymer composition — p-(VdF/TFE) = (VdF/TFE) = layer 80/20wt % 80/20 wt %, MI = 120 (protect. of the outermost periphery)Thickness (μm) — 10 5% (Note 1) Nozzle temperature 220 210 220 220 (°C.) Example 20 Comparative Example 8 Comparative Example 9 Core number 11 1 Core 1 Composition MMA MMA MMA Initiator V601 V601 V601 Initiatorconc. (*10⁻⁵ 1.8 1.8 8.7 mol/1 mol monomer) Polymerization temp. 130 110130 to 140 (° C.) (unstable) Mercaptan, conc. (*10⁻³ OcSH n-BtSH n-BtSHmol/1 mol monomer) 1.2 1.8 1.8 Average residence time 3.6 3.6 1 (hrs)Polymer content (wt %) 45 42 to 50 40 to 50 (unstable) (unstable)Residual monomer (MMA, 0.24 wt %) Residual monomer (4FM, wt %) Boundsulfur component 585 (ppm) Non-bound sulfur com- 87 ponent (ppm)Diameter (μm) Core 2 Composition MMA/4FM = MMA/4FM = MMA/4FM = 80/20 wt% 80/20 wt % 80/20 wt % Initiator V601 V601 V601 Initiator conc. (*10⁻⁵1.8 1.8 8.7 mol/1 mol monomer) Polymerization temp, 130 110 130 to 140(° C.) (unstable) Mercaptan, conc. (*10⁻³ OcSHn-BtSH n-BtSHn-BtSH n-BtSHmol/1 mol monomer) 1.2 1.8 1.8 Average residence time 3.6 3.6 1 (hrs)Polymer content (wt %) 47 42 to 50 40 to 55 (unstable) (unstable)Residual monomer (MMA, 0.14 wt %) Residual monomer (4FM, 0.11 wt %)Bound sulfur component 580 (ppm) Non-bound sulfur com- 102 ponent (ppm)Thickness (μm) Core 3 Composition — — — Initiator — — — Initiator conc.(*10⁻⁵ — — — mol/1 mol monomer) Polymerization temp. — — — (° C.)Mercaptan, conc. (*10⁻³ — — — mol/1 mol monomer) Average residence time— — — (hrs) Polymer content (wt %) — — — Residual monomer (MMA, — — — wt%) Residual monomer (4FM, — — — wt %) Bound sulfur component — — — (ppm)Non-bound sulfur com- — — — ponent (ppm) Thickness (μm) — — Core 4Composition — — — Initiator — — — Initiator conc. (*10⁻⁵ — — — mol/1 molmonomer) Polymerization temp. — — — (° C.) Mercaptan, conc. (*10⁻³ — — —mol/1 mol monomer) Average residence time — — — (hrs) Polymer content(wt %) — — — Residual monomer (MMA, — — — wt %) Residual monomer (4FM, —— — wt %) Bound sulfur component — — — (ppm) Non-bound sulfur com- — — —ponent (ppm) Thickness (μm) — — — Sheath Copolymer composition17FM/MMA/MAA = 28/71/1 wt % Thickness (μm) Protect. Copolymercomposition layer Thickness (μm) Nozzle temperature 220 220 220 (° C.)

[0246] TABLE B Increase Transmission in heat/ loss (dB/km) Transmissionmoisture Example Radical 520 570 band resistance No. initiator nm nm 650nm (MHz) −50 m (dB/km) 16 (A) 70 62 119 550 30 17 (A) 68 60 121 820 3018 (A) — — 139 800 32 19 (B) 85 74 133 — 60 20 (A) 92 85 136 200

1. A process for manufacturing a methacrylate (co)polymer comprisingconducting polymerization while feeding a monomer (mixture) containingat least 90 wt % in total of at least one methacrylate monomer and aradical polymerization initiator represented by formula (II) into areactor, where an initiator concentration and a polymerizationtemperature satisfy a relationship represented by equations (1) to (4)and the polymerization temperature is not less than 110° C. and not morethan 160° C.; ln(A)≦105.4−45126/B   (1) ln(A)≦2545.2/B−15.82   (2)ln(A)≧225.9−102168.8/B   (3) ln(A)÷1300.0/B−15.74   (4) wherein A is aninitiator concentration (a molar ratio of the initiator/the monomer); Bis a polymerization temperature (° K); and 1n is a symbol for a naturallogarithm;

wherein R is alkyl or fluoroalkyl.
 2. The process as claimed in claim 1,where an inert solvent is further fed to the reactor in thepolymerization step and instead that the initiator concentration and thepolymerization temperature satisfy the relationship represented by theabove equations (1) to (4), the initiator concentration, thepolymerization temperature and an inert solvent concentration satisfy arelationship represented by equations (5) to (8):ln{A×(1−C)⁵}≦105.4−45126/B   (5) ln{A×(1−C)⁵}≦2545.2/B−15.82   (6)ln{A×(1−C)⁵}≧225.9−102168.8/B   (7) ln{A×(1−C)⁵}≧1300.0/B−15.74   (8)wherein A is an initiator concentration (a molar ratio of theinitiator/the monomer); B is a polymerization temperature (° K); C isthe concentration of the inert solvent (the amount of the inert solvent(g)/the total amount of the monomer, the initiator, the chain transferagent and the inert solvent fed into the reactor (g)); and In is asymbol for a natural logarithm.
 3. The process as claimed in claim 1,where the monomer (mixture) contains at least one monomer selected fromthe group consisting of methyl methacrylate, a fluoroalkyl methacrylateand benzyl methacrylate.
 4. The process as claimed in claim 1, where inthe polymerization step, methyl methacrylate is used as one methacrylatemonomer, the content of methyl methacrylate in the monomer (mixture) isat least 80 wt %, and the compound represented by formula (III) is usedas a radical polymerization initiator:


5. The process as claimed in claim 1, further comprising a feeding stepof feeding a reaction mixture taken out from the reactor to adevolatilization step and a devolatilization step of separating andremoving volatiles from the reaction mixture.
 6. The process as claimedin claim 4, further comprising a feeding step of feeding a reactionmixture taken out from the reactor to a devolatilization step and adevolatilization step of separating and removing volatiles from thereaction mixture.
 7. The process as claimed in claim 5, where in thepolymerization step, the monomer (mixture) and the initiator arecontinuously fed to the reactor for bulk polymerization and in thefeeding step, the reaction mixture is continuously fed from the reactorto the devolatilization step.
 8. The process as claimed in claim 5,where a polymer content in the reaction mixture in the polymerizationzone is 30 wt % to 70 wt % both inclusive.
 9. The process as claimed inclaim 5, where in the polymerization step an alkyl mercaptan having 3 to6 carbon atoms is further fed to the reactor for conductingpolymerization.
 10. The process as claimed in claim 5 further comprisinga volatile purification step, where the volatiles separated and removedin the devolatilization step are purified using a catalyst containing atleast one element selected from the group of copper, cobalt, nickel andmanganese in the presence of molecular oxygen and further in thepresence of a compound containing at least chlorine.
 11. A process formanufacturing an optical fiber comprising feeding the (co)polymerprepared by the process as claimed in claim 5 and another polymer havinga different refractive index to a multi-component spinning nozzle forspinning.
 12. A process for manufacturing an optical fiber comprisingfeeding at least two (co)polymers mutually different in a copolymercomposition and in a refractive index prepared by the process as claimedin claim 5 to a multi-component spinning nozzle for spinning byconcentrically piling the polymers in a manner that a refractive indexis reduced from the center toward the periphery.
 13. A process formanufacturing an optical fiber comprising feeding a core materialcomprising a (co)polymer prepared by the process as claimed in claim 1with other (co)polymer to a multi-component spinning nozzle for spinningby assembling a plurality of islands, where each of the islands has acore and the islands are separated from each other by other (co)polymer.14-27. (Canceled).